The use of Raman or other scattering for purposes of tailoring a light beam pulse (tuning, amplification, pulse shaping) has been considered occasionally in the literature. One peripheral use is described in U.S. Pat. No. 3,516,744, issued to Hinman, Slomba and Stoddart for a Raman laser beam sampling cell or spectrometer, wherein the beam makes multiple passes through a Raman sampling cell and the resulting Raman radiation is collected at a non-zero angle (e.g., 90.degree.) relative to the beam propagation direction. The scattered radiation thus measured is not used for any other purpose, and the multiple passes of the beam through the cell are not synchronized.
A laser pulse compression or shaping system is taught by Brienza and Treacy in U.S. Pat. No. 3,549,256, wherein a negatively chirped pulse and a positively chirped pulse are generated (of equal intensity, but differing in phase) by means of a rotating mirror forming one end of two side-by-side optical cavities that generate the two initially similar pulses. Using a laser gas with a narrow spectral range so that only one mode is within the gas gain band at any instant, Brienza et al find that single dominant mode present in the combined chirped pulses sweeps across the gain bandwidth in a time of order of 1 .mu.sec for mirror rotations of the order of 30 cps; and this limits the length of an output pulse from the system, possibly resulting in pulse compression.
Pantell and Puthoff, in U.S. Pat. No. 3,624,421, disclose and claim a tunable Raman laser, using a crystal that is both Raman active and infrared active and has a preferred axis that is controllably oriented at a non-zero angle .theta. relative to the axis of the laser pump, with the crystal preferred axis coinciding with the optical cavity in which the crystal is positioned. By suitable choice of the angle .theta., the laser pump (at the Raman frequency) generates first order Stokes radiation propagating in the crystal parallel to the optical cavity axis, and this Stokes radiation is amplified by successive passes through the crystal. As .theta. is varied, the output frequency from the optical cavity varies continuously, thus affording a tunable Raman laser. The length of the Raman pump pulse is not controlled, each such pulse makes only one pass through the crystal, and passage of the Raman pump pulse and Stokes radiation through the crystal are not synchronized.
U.S. Pat. No. 3,657,554 to Lumpkin and Shiren teaches a technique for generating super-radiant, short laser pulses, utilizing adiabatic rapid passage of an optical transition (electric dipole) in atoms in an optically resonant medium. The invention requires (1) that the atomic population be substantially fully inverted and (2) that the seed pulse duration be much shorter than the relaxation time of the medium so that little spontaneous decay occurs. The (giant) pulse produced by such an amplifier is then passed through an electro-optical crystal such as LiNbO.sub.3 where the carrier frequency is modulated so as to excite Raman transitions in fluoro-nitrobenzene, contained in a Raman cell through which the pulse then passes. Super-radiant laser pulses then issue from the Raman cell, the result of adiabatic rapid passage of the 4 .sup.2 S.sub.1/2 .fwdarw.4 .sup.2 P.sub.3/2 transition in the Raman cell. No special use is made of any Stokes radiation generated, and the system is not a multipass or regenerative laser system.
A technique for compression of optical laser pulses is taught by U.S. Pat. No. 3,720,884 issued to Kelley, Fisher and Gustafson. A short optical pulse is passed through a Kerr cell, having a nonlinearity relaxation time&lt;&lt;the seed pulse duration, .DELTA.t; and the Kerr cell material produces a positive chirp on the optical pulse passing therethrough that spreads the frequency band of the unmodulated pulse, which is no longer monochromatic, thereby temporally compressing the optical pulse to perhaps 10.sup.-13 sec. Here, according to the inventors, the stimulated Raman effect is to be avoided as it interferes with self-phase modulation in the Kerr cell.
A laser system employing anti-Stokes radiation, which is a concomitant of Raman scattering, for production of high power laser pulses is disclosed and claimed in U.S. Pat. No. 3,815,043, issued to Carman and Rhodes. In one embodiment, an oscillator produces radiation at an initial wavelength .lambda..sub.1, which is amplified by a first amplifier and directed through an electron beam-pumped Raman cell to produce anti-Stokes radiation of wavelength .lambda..sub.2 &lt;.lambda..sub.1 by forward Raman scattering. The input radiation is directed through the Raman cell at a time (approximately 1 .mu.sec after electron beam pumping begins) corresponding to maximum inversion density in the Raman cell medium (e.g., CO.sub.2 gas) so that maximum stored energy is available for Raman scattering and Stokes conversion in the gas. The process requires use of a high intensity, short duration (.DELTA.t.ltorsim.0.3 nsec) input pulse to efficiently convert part of the stored Raman cell energy to, say, first order anti-Stokes radiation that then propagates at a predetermined angle relative to the propagation direction of the input pulse. This system does not contemplate multipass or regenerative use of either the input pulse or the anti-Stokes radiation pulse thus produced, and it requires use of a short duration (.DELTA.t.ltorsim.0.3 sec) input pulse for efficient operation.
U.S. Pat. No. 4,039,851, issued to Jain, Lin and Stolen teaches the use of optically dispersive Raman scattering in an optical fiber to disperse (in time) a Stokes radiation pulse produced in the fiber. The frequency- and temporally-dispersed Stokes radiation pulse is multiply passed through the fiber in synchronism with additional seed pulses produced by a laser pump, and pulse components with the particular frequency desired are enhanced relative to all other frequencies. The invention uses synchronized multiple passes of the Stokes pulse primarily for dispersion discrimination in favor of one frequency or narrow band of frequencies, rather than for energy amplification or transfer of energy from one wavelength to another.